The invention relates to a method for determining the filling pressure for a clutch that comprises clutch linings, a clutch release spring, and a hydraulic pressure system for closing the clutch against the force of the clutch release spring, wherein at an engagement point of the clutch the clutch linings abut against each other and any further increase in pressure in the pressure system immediately creates the capacity of the clutch in transmitting a torque.
Usually, the pressure system comprises of a clutch cylinder and a clutch piston that are rigidly connected with an input side or an output side of the clutch and rotate with this clutch. At an engagement point of the clutch the force from the clutch piston to the clutch linings and the force from the clutch release spring equalize each other so that the clutch linings are in abutment, but without being subjected to any force pressing the clutch linings against each other.
In motor vehicle design it becomes more and more common to use clutches in the dual clutch transmissions that are automatically controlled by a control apparatus. In that kind of control, specific knowledge of the engagement point is crucial for the quality of operating the clutch.
For reaching from an entirely disengaged or open clutch this engagement point, the clutch cylinder has to be filled with hydraulic fluid. For effecting this filling process the filling pressure can be set to a high value. By a high filling pressure a high volume flow can be set in that pressure system so that the clutch cylinder is filled quickly and the engagement point, starting from the open clutch, can be reached in a short period of time. However, this results in the risk that the high filling pressure when filling the clutch cylinder causing the clutch piston to exert a force on the clutch linings is higher than the counterforce exerted by the clutch release spring, resulting in that the clutch linings are not only in abutment, but are actually pressed against each other, causing the clutch to have the capacity to transmit torque, which is not desired at this point in time. For control of a clutch that is for instance used in a motor vehicle, this is detrimental to the driving comfort.
If in contrast a comparatively low filling pressure is set, filling the clutch cylinder and therefore reaching the engagement point would cost too much time, likewise resulting in a detrimental effect on the gear shifting comfort.